The invention relates to the field of wavelength multiplexed optical communications systems, such as wavelength division multiplexed (WDM) systems, and more particularly to the management and control of the power level of wavelength multiplexed optical signals.
Optical communications systems are a substantial and fast-growing constituent of communications networks. The expression xe2x80x9coptical communications system@, as used herein, relates to any system which uses optical signals to convey information across an optical medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems and local area networks (LANs). Optical systems are described in Gower, Ed. Optical Communications Systems, (Prentice Hall, N.Y.). Currently, the majority of optical communication systems are configured to carry a single optical channel having a narrow wavelength spectral band over one or more optical waveguides. To convey information from a plurality of sources, wavelength division multiplexing (WDM) is now used. In a WDM system a plurality of optical signals, each typically having a narrow wavelength spectral band, each band being centered on a different wavelength, are carried over a single optical guide.
A typical optical network comprises a plurality of nodes linked by a number of different optical paths for the carriage of WDM signals therebetween. Typically, each path will introduce a different amount of attenuation into the signal components passing through it.
One problem experienced in optical communication systems is the wide variation in the attenuation experienced by optical signals following different routes through the network. This leads to a corresponding variation in the power levels of different signals arriving at a particular node in an optical network via different routes. Such variation is experienced even with signals originating from the same point in the network with nominally identical power levels, but following different paths between nodes. In order to overcome this problem there is a need for a means to selectively attenuate optical signals arriving at a point in the network.
A prior art method for applying a different attenuation to each WDM signal arriving at a certain node in an optical network has been to demultiplex all of the channels onto separate paths by means of a wavelength demultiplexer using interferometers, diffraction gratings or waveguide phasor devices to insert waveguide attenuators into each of the separate paths, (thus allowing for each channel to be attenuated independently of the others) and to then recombine all of the channels using a multiplexer of similar principle to that of the demultiplexer. This method has the disadvantage that it involves significant insertion loss in the optical path because of the number of components required. The component parts and the assembly required by this method results in a high cost and a complexity that increase with the number of channels.
The present invention provides a system for controlling the power levels of a first plurality of optical signals in a first optical path; in which the optical signals are spectrally separated; in which the, optical path comprises optical attenuation means comprising a first plurality of variable optical attenuators, each optical attenuator for attenuating in a different part of the spectrum.
In a preferred embodiment, the present invention provides a system comprising a second optical path, the system for also controlling the power levels of a second plurality of optical signals in the second optical path; in which the second plurality of optical signals form a set with the first plurality of optical signals, and in which the optical signals of the set are spectrally separated; in which the second optical path comprises a second plurality of optical attenuators and in which the second plurality of optical attenuators form a set with the first plurality of optical attenuators of, each optical attenuator of the set for attenuating in a different part of the spectrum; in which, in part, the first and second paths share a first optical guide; in which the system comprises an interferometer for separating the second plurality of optical signals in the shared part of the first optical guide into a second optical guide of the second path; in which the first and second optical guides comprise the first and second pluralities of optical attenuators respectively.
In a further preferred embodiment, the present invention provides a system in which the optical attenuation means comprises a further plurality of optical attenuators each optical attenuator of the further plurality for attenuating a different part of the spectrum; each optical attenuator of the further plurality for attenuating the same part of the spectrum as, and comprised in the same path as, a corresponding optical attenuator of the first plurality.
In a further preferred embodiment, the present invention provides a system for selectively delaying components of an optical signal according to the wavelengths of the components.